The vertebrate immune system requires multiple molecular and cellular interactions to achieve optimal immune responses. In particular, activation of T lymphocytes (T cells) is an important component of many such responses. Antigen-presenting cells (APC) can activate T cells by presentation of antigens via peptides carried by major histocompatibility complex (MHC) molecules to the TCR (T cell receptor). Such activation also requires co-stimulation by APC. Delivery of a non-specific costimulatory signal to T cell requires at least two homologous B7 family members found on APC, B7-1 (also called B7, B7.1, or CD80) and B7-2 (also called B7.2 or CD86), both of which can deliver costimulatory signals on binding to the CD28 antigen on T cells resulting in T cell activation. CD28 is a homodimeric glycoprotein member of the immunoglobulin (Ig) superfamily with a single extracellular variable region, and is present on most mature human T cells.
A homologue of CD28 named CTLA4 (Cytotoxic Lymphocyte Associated Antigen, also designated CD152) was discovered in 1987 (Brunet et al., (1987) Nature 328:267-270) with particular association with cytotoxic T cells. As with CD28, CTLA4 is a member of the Ig superfamily and comprises a single extracellular Ig domain. However, the role of CTLA4 is primarily to inhibit T cell activation and this was shown in CTLA4 deficient mice (Chambers et al., (1997) Immunity. 7:8855-8959) which suffer from massive lymphoproliferation. In addition, blockage of CTLA4 was shown to enhance T cell responses in vitro (Walunas et al., (1994)) Immunity. 1:405-413 and in vivo (Kearney (1995) J. Immunol. 155:1032-1036) and also to increase antitumour immunity (Leach (1996) Science. 271:1734-1736). Therefore, blockage of CTLA4 might provide new treatments for disease, especially human diseases where immune stimulation might be beneficial such as for treatment of cancers and infectious diseases.
Development of blockers of CTLA4 function has focused on the use of monoclonal antibodies, especially antibodies derived from transgenic mice engrafted with genes encoding human immunoglobulins (and deficient in host mouse immunoglobulin genes). Clinical trials are ongoing with such antibodies including Ipilimumab (Keler et al., J Immunol 171:6251-6259 (2003)), which is an IgG1 isotype, and Tremelimumab (Ribas et al., Oncologist 12: 873-883 (2005)) which is an IgG2 isotype. Whilst the immunogenicity (induction of antibodies against the injected human monoclonal antibodies) is generally reported to be low, there is concern that such human antibodies, due to somatic mutations and rearrangements in the variable region sequences (which may result in T cell epitopes), may induce immunogenicity in some patients resulting in adverse effects and lack of therapeutic effect. There is thus a need for improved anti-CTLA4 monoclonal antibodies with a potentially lower immunogenicity in order to provide more effective treatments of human diseases.